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wrf_user_ij_to_ll

Finds the nearest longitude, latitude locations to the specified model grid indices (i,j).

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"

	function wrf_user_ij_to_ll (
		file_handle  : file or list [list added in V6.0.0],  
		i            : numeric,                              
		j            : numeric,                              
		opt          : logical                               
	)

	return_val  :  float or double

Arguments

file_handle

Reference to an input netCDF file opened with addfile, or, as of V6.0.0, can be a list of NetCDF files opened with addfiles.

i
j

Arrays of I and J indexes for which longitude and latitude locations are required. These index values are Fortran-based, so the index count starts at 1 (and not 0 like it does in NCL).

opt

A logical scalar containing attributes. See description below.

Return value

An array that holds the lon, lat locations to the input i, j indexes. If a single i, j location is used as input, the return value will have a dimension of 2, where the first element holds the lon location and the second element holds the lat location.

If an array of i, j values are used on input, the return value will be 2 x n, where n is the size of the input arrays.

Description

This function makes use of map projection information out of the input file (nc_file) to locate the lon/lat locations which corresponds to the input i/j locations. Note that if the i/j locations are outside your model domain, the returned lon/lat locations will represent a point outside the domain. These index values are Fortran-based, so the index count starts at 1 (and not 0 like it does in NCL).

  • useTime - which time in the file should be used when extracting XLAT/XLONG arrays from input file. Only important to set if output is for a moving nest and all the output times are in a single wrfout file
wrf_user_ij_to_ll is part of a library of functions and procedures in WRFUserARW.ncl written to help users plot ARW WRF model data.

Questions on this function should be sent to wrfhelp@ucar.edu.

See Also

wrf_user_ll_to_ij, wrf_ll_to_ij, wrf_ij_to_ll, wrf_user_getvar, wrf_user_intrp2d, wrf_user_intrp3d, wrf_user_list_times, wrf_user_unstagger

See the full list of WRF functions.

Examples

Example 1


  load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
  load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"

  a   = addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")
  loc = wrf_user_ij_to_ll(a, 10., 30., True)
  print("lon location is: " + loc(0))
  print("lat location is: " + loc(1))

Example 2

This example shows how using the output from wrf_user_ll_to_ij as input to wrf_user_ij_to_ll should give you close to the same values:

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"

  a    = addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")
  xlat = a->XLAT(0,:,:)
  xlon = a->XLONG(0,:,:)

  minlat =  30
  maxlat =  40
  minlon = -80
  maxlon = -70

;---Get ij indexes, and then use these to get lat/lon values back again.
  opt    = True
  loc    = wrf_user_ll_to_ij(a,(/minlon,maxlon/),(/minlat,maxlat/),opt)
  latlon = wrf_user_ij_to_ll(a,loc(0,:),loc(1,:),opt)


;---The min/max values printed should be close.
  print("----")
  print("Requested  min/max xlon = " + minlon + "/" + maxlon)
  print("Actual     min/max xlon = " + xlon(loc(1,0)-1,loc(0,0)-1) + "/" + \
                                       xlon(loc(1,1)-1,loc(0,1)-1))
  print("Calculated min/max xlon = " + latlon(0,0) + "/" + latlon(0,1))

  print("----")
  print("Requested  min/max xlat = " + minlat + "/" + maxlat)
  print("Actual     min/max xlat = " + xlat(loc(1,0)-1,loc(0,0)-1) + "/" + \
                                       xlat(loc(1,1)-1,loc(0,1)-1))
  print("Calculated min/max xlat = " + latlon(1,0) + "/" + latlon(1,1))
  print("----")

Example 3

As of NCL V6.0.0, you can retrieve ij coordinates from a list of WRF ARW NetCDF files:

  load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
  load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"

  files = systemfunc("ls -1 wrfout_d01_2000*") + ".nc"
  a = addfiles(files,"r")

  loc = wrf_user_ij_to_ll(a, 10., 30., True)
  print("lon/lat locations are: " + loc)

You can see some other example scripts and their resultant images at:

http://www.mmm.ucar.edu/wrf/OnLineTutorial/Graphics/NCL/